The present invention relates to magnetostatic wave devices, in general, and more particularly to a magnetostatic wave device which includes a structure for causing a variation in the propagation velocity of the magnetostatic waves supported by the device.
In general, magnetostatic waves are supported by a magnetized medium which in most cases is a thin ferrite layer or slab under the effects of a magnetic field. The propagation of the magnetostatic waves supported by the magnetized medium is measured by its wave number or group velocity, or in reciprocal form, the group delay times. In one experimental arrangement, conductors were disposed at the ends of the magnetized ferrite slab, which was a single crystal of yttrium-iron-garnet (YIG) and an electromagnetic signal was conducted through the conductor at one end, commonly referred to as the generator, causing a magnetostatic wave to propagate across the YIG layer at a propagation velocity. Accordingly, an electromagnetic signal is induced in the conductor at the other end of the ferrite layer by the magnetostatic waves received thereby, this conductor being commonly referred to as the receiver. Thus, the group delay time of the magnetostatic waves may be measured as the time between generation and reception across the propagation path of the ferrite layer. For a more detailed understanding of the phenomenon of magnetostatic wave propagation as supported by a magnetized medium, reference is hereby made to an article in the IEEE Proceedings, 58, 506, (1970) authored by S. R. Seshadri. In the Seshadri experiments, it has been observed that when one surface of the magnetized medium is metalized, the propagation velocity of the magnetostatic waves is at least twice that of when the surface is free of metal, commonly referred to as a free surface.
In another set of experiments, referred to as the Castera/Hartemann experiments, magnetostatic surface waves were propagated in a thin YIG film grown on a &lt;111&gt; oriented gadolinium gallium garnet (GGG) substrate. A magnetic field was applied in the plane of the YIG film perpendicular to the direction of wave propagation. An array comprising a plurality of metallic strips was disposed on one surface of the film separated by less than one half a wavelength and aligned transverse to the direction of wave propagation. Generator and receiver elements were disposed at respective ends of the propagation path on the surface of the film. It was observed in these experiments that the propagation velocity of the magnetostatic surface waves influenced by the isolated metallic strips is close to the wave velocity on a free surface. However, when the metallic strips were electrically connected together at both ends thereof, the magnetostatic surface wave propagation velocity became close to that of waves propagating under the influence of a metalized surface. Exemplary observed dispersion relationships taken from the Castera/Hartemann experiments are shown in the graph of FIG. 1A. In FIG. 1A, the solid lines 20 and 22 represent the dispersion relations for metalized surface and free surface, respectively, as developed through the Seshadri experiments. The dashed lines 24 and 26 are representative of the dispersion relations for open strips and shorted strips as developed through the Castera/Hartemann experiments.
FIG. 1B depicts a graph illustrating the group velocity versus signal frequency relationships of magnetostatic surface waves between open and shorted metallic surface strips as estimated from the dispersion relationships shown in FIG. 1A. The solid lines 28 and 30 are representative of the estimated group velocities over the designated signal frequency range for the shorted and open metallic surface strips, respectively. For a more detailed understanding of these experimental results, reference is hereby made to an article in Electronics Letters 16, 195, (1980) authored by J. P. Castera and P. Hartemann.